Tamerlan Tsarnaev, the slain man suspected of carrying out the Boston Marathon bombing with his younger brother, told a relative he was seeking a copy of the Protocols of the Elders of Zion.
Elmirza Khozhugov, who was married for a time to Tsarnaev's sister, Ailina, told the Associated Press in an interview published Tuesday that Tsarnaev had been radicalized by a Muslim convert whom he named as Misha.
Tsarnaev started visiting conspiracy websites, Khozhugov said, and was seeking a copy of the Protocols, a notorious Tsarist forgery positing world control by a Jewish cabal.
Federal authorities have charged Tsarnaev's younger brother and alleged accomplice, Dzhokhar, with a weapos of mass destruction charge for alleged role in planting the bombs that killed three people and wounded scores in last week's marathon.
The elder Tsarnaev was killed in a shootout last Friday, but not before a policeman was killed during a day-long chase. It is still not clear what motivated the attacks.
The Tsarnaevs are ethnic Chechens, a persecuted minority in Russia.